1. Field of the Invention
The present invention relates to an electron emission source, an electric device using the same, and a method of manufacturing the electron emission source, and more particularly, to an electron emission source using a needle-shaped electron emission material such as carbon nanotubes (CNT).
2. Description of the Related Art
Carbon nanotubes (CNTs) or nanoparticles are preferred as electron emission materials of electron emission sources. CNTs refer to tubular molecules composed primarily of carbons. There are various types of CNTs according to shapes. CNTs have very good electrical, mechanical, chemical, and thermal properties, and thus are applied to various fields. CNTs have a low work function and a high aspect ratio. Since the radius of curvature at a top end or emission end is small, CNTs have a very high field enhancement factor, thereby making it possible to emit electrons at a low electric field.
Conventional methods of manufacturing a CNT electron emission source are roughly divided into a method of vertically growing CNTs directly on a conductor, such as a cathode or a substrate, and a method of attaching CNT powder, which is separately synthesized, to a cathode.
Examples of the method of vertically growing the CNTs directly on the conductor includes a lot of methods that involve vertically aligning CNTs on various cathode substrates, on which catalytic metal particles are deposited, through decomposition of carbon source gas at high temperature (refer to Science vol. 283, 512, 1999; Chemical Physics Letters. 312, 461, 1999; Chemical Physics Letters. 326, 175, 2000; Nano Letter vol. 5, 2153, 2005; US006350488B1; and US006514113B1).
Examples of the method of attaching the synthesized CNT powder to the cathode include suspension filtering, screen printing, electrophoresis, self-assembly, spraying, and inkjet printing.
A suspension filtering method involves filtering a CNT suspension through filter paper having pores and transferring the filtered CNT suspension to a cathode substrate coated with Teflon (refer to Science vol. 268, 845, 1995, and Applied Physics Letters vol. 73, 918, 1998).
A screen printing involves printing and firing paste, which is formed by mixing CNT powder with a vehicle containing a polymer and an organic solvent, an inorganic binder, and other additives, on a cathode substrate to form a CNT thin film (refer to Applied Physics Letters vol. 75, 3129, 1999, and Korean Patent Publication No. 10-2007-0011808).
An electrophoresis method involves loading a cathode substrate in an electrolyte solution containing a surfactant and CNT powder and attaching CNT particles to the cathode substrate by using electrophoresis (refer to Advanced Materials vol. 13, 1770, 2001; Nano Letter vol. 6, 1569, 2006; US006616497B1; and US200600555303A1.
A self-assembly method involves vertically dipping a hydrophilic substrate in a suspension where CNTs whose surfaces are modified to hydrophilic are dispersed in deionized water to form a CNT thin film through slow evaporation (refer to Advanced Materials vol. 14, 8990, 2002; and US006969690B2).
A spraying method involves spraying an evenly dispersed CNT suspension through a spray nozzle to form a CNT thin film on a cathode substrate (refer to Mat. Res. Soc. Symp. Proc. vol. 593, 215, 2000; Carbon vol. 44, 2689, 2006; the Journal of Physical Chemistry C. 111, 4175, 2007; US006277318B1; and Korean Patent Publication No. 10-2007-0001769).
An inkjet printing method involves printing an evenly dispersed CNT suspension on a cathode substrate by using an inkjet printer to form a CNT thin film (refer to Small. vol. 2, 1021, 2006; Carbon vol. 45, 27129, 2007; and US20050202578A1).
In detail, a method of directly vertically growing CNTs comprises depositing a nano-sized catalytic metal on a conductive or non-conductive cathode substrate through sputtering, thermal deposition, electron (E)-beam evaporation, or the like, thermally decomposing carbon source gas, that is, a gaseous or liquid hydrocarbon, at high temperature through chemical vapor deposition (CVD), and manufacturing an vertically aligned CNT field electron emission source. This method has advantages in that it is easy to control the diameter, length, density, and pattern of the CNTs, but has disadvantages in that it is difficult to ensure high uniformity and control the particle size of the catalytic metal when the catalytic metal is deposited over a large area, adhesion between the grown CNTs and the cathode substrate is weak, and it is not easy to manufacture a large CNT field electron emission source.
In order to solve the weak adhesion between the CNTs and the cathode substrate and the difficulty in manufacturing the large CNT field electron emission source, various methods of purifying, dispersing, and functionalizing synthesized CNT power into paste or dispersing synthesized CNT in a solvent and a surfactant to form a suspension and attaching CNTs to a cathode substrate have been developed. Among the various methods, a screen printing method of printing CNT paste, which includes CNT powder, a polymer, a binder, an organic solvent, a metal filler, and other additives, on a cathode substrate and manufacturing a CNT electron emission source through drying, exposure, firing, surface protrusion process, and so on has advantages in that adhesion between the cathode substrate and the CNT electron emission source is strong and a large CNT electron emission source can be manufactured, but has disadvantages in that it is difficult to control the density of an active electron emission site, field electron emission characteristics are easily deteriorated due to the variety of organic and inorganic binders and polymers, and a manufacturing process is complicated. An electrophoresis method of mixing CNT powder with a dispersing agent in an electrolyte solution to form an evenly dispersed CNT suspension, loading two electrode substrates in the CNT suspension to form an electric field, depositing CNTs positively charged in the electric field on a cathode substrate to which a negative voltage is applied to manufacture a CNT field electron emission source has advantages in that selective deposition can be made at room temperature and a large CNT field electron emission source can be manufactured, but has disadvantages in that it is difficult to control thickness and density, uniformity and reproduction are poor, and adhesion between the CNTs and the cathode substrate is weak, thereby reducing reliability and stability during field electron emission.
A self-assembly method of vertically dipping a hydrophilic cathode substrate in a suspension where CNTs whose surfaces are modified to hydrophilic are dispersed in deionized water to form a CNT field electron emission source through slow evaporation has advantages in that a manufacturing process is simple and the CNT field electron emission source can be easily made large at room temperature, but has disadvantages in that adhesion between a CNT thin film and the cathode substrate is weak, like the electrophoresis method, and lots of time is required.
A spraying method has advantages in that a manufacturing process is simple and a large CNT field electron emission source can be easily manufactured at room temperature, but has disadvantages in that, since the state of a surface of a CNT thin film is determined by the amount of suspension that evaporates while the suspension is sprayed from a nozzle to a cathode substrate, it is difficult to control the thickness and density of the CNT thin film, it is also difficult to uniformly deposit the CNT thin film, which results in low uniformity and reproduction, and adhesion between the CNT thin film and the cathode substrate is weak, which leads to easy detachment during electric field electron emission.
An inkjet printing method of selectively printing a suspension, which is formed by evenly dispersing CNT powder whose surface is modified to hydrophilic in deionized water, on a cathode substrate to form a CNT field electron emission source had advantages in that it is easy to control the thickness and density of a CNT thin film, and the CNT thin film can be selectively patterned and can be made large at room temperature, but has disadvantages in that adhesion between the printed CNT field electron emission source and the cathode substrate is weak. A suspension filtering method of filtering an evenly dispersed CNT suspension through filter paper having pores, and simply transferring the filtered CNT suspension to a cathode surface coated with Teflon to form a CNT field electron emission source has advantages in that it is easy to control the thickness and density of a CNT thin film by controlling the amount or density of CNT powder, a manufacturing process is simple, and a large CNT field electron emission source can be manufactured, but has disadvantages in that adhesion between the CNT thin film and the cathode substrate is weak.
As a modification of the suspension filtering method of forming the CNT thin film and then transferring the CNT thin film to the cathode substrate, a method of bonding a CNT thin film, which is directly grown and vertically aligned, to a layer where conductive silver paste is patterned, thermally compressing the CNT thin film, and transferring the resultant CNT thin film to a metal substrate, or preparing a patterned conductive layer on a glass sheet, depositing conductive carbon paste, such as, silver or gold paste, on the conductive layer, and transferring CNTs, which are moved from a CNT thin film, which is directly grown and vertically aligned, to an adhesion sheet, to the conductive paste deposited on the conductive layer to form a CNT field electron emission source is disclosed in US 2004/0166235A1. However, this method has disadvantages in that it is difficult to manufacture a large CNT thin film because the CNT thin film is directly grown and vertically aligned, and a manufacturing process is complicated because drying, compression, and heating, or thermal compression, should be performed to ensure high adhesion when the CNTs are transferred.
In manufacturing a good CNT electron emission source, high reliability, high stability, and low cost should be ensured. Impurities badly affecting electron emission should not be mixed. The density of CNTs should be easily controlled for high uniformity and reproduction. Adhesion between the CNTs and a cathode supporting the CNTs should be high enough to ensure reliability and stability of the CNT electron emission source. Also, a manufacturing processes should be simple to reduce manufacturing costs and a large CNT electron emission source should be able to be manufactured.